In a traditional prime mover system, one goal of the engine, gearbox and generator is to provide a DC distribution system that can maintain a terminal voltage in a relatively narrow range. At the same time, the prime mover should operate at a variable speed in order to have fuel-efficient performance. With a conventional salient pole synchronous machine (i.e. wound-rotor machine), the excitation system and a conventional three-phase (i.e. six pulse) rectifier provide this capability. However, the wound-rotor system configuration is incompatible with high-speed prime movers without a gear-reduction stage. Therefore, a gearless-oilless system is not attainable. Also, such a wound-rotor machine has poor reliability and low efficiency.
In contrast, the rotor construction of permanent magnet (PM) generators is robust and allows for high-speed operation, but excitation by definition is fixed. Therefore, a full-scale three-phase bridge or active power electronics with similar complexity is required in order to condition the variable-voltage power provided by the PM machine. However, three-phase switching bridges are very expensive for high-power applications. They also present reduced reliability and increased volume and weight, and the losses in the switching devices can be substantial.
One solution to providing regulated voltage using a PM machine is described in U.S. Pat. No. 5,245,238 issued to Lynch et al., the entire content of which is incorporated herein by reference. As noted in the Lynch Patent, an axial air gap permanent magnet generator having more than one rotor or stator is provided. The voltage regulation in this generator is achieved by controlling the mutual position between multiple rotors or multiple stators using some form of actuation. The requirement for multiple rotors or multiple stators with mutual position change however increases the complexity of the generator and reduces reliability. The need for additional actuation hardware further adds to the complexity of the system.
Still another solution including the use of permanent magnet generators is described in U.S. Pat. No. 6,087,750 issued to Bernard Raad, the entire content of which is incorporated herein by reference. The Raad Patent discloses the use of a permanent magnet generator for a very specific application that restricts diametrical space, such as oil exploration wells, and the type of machine described is of an axial air gap type only. In addition to the main generator windings, there are two more windings used for regulation of the output voltage, which, in the generator disclosed in the Raad Patent, is achieved in two ways. First, the regulation coil acts by saturating the iron core with DC flux whenever the output voltage exceeds a pre-selected limit due to variations in load and speed. The regulation coil creates a flux which is opposed to that of the magnetic field assembly. Secondly, the regulation coil also regulates the output voltage by acting as an added load on the main power output of the generator.
In the Raad Patent, the regulation coils are placed in the air gap between the magnet rotor and the stator teeth of the iron core. However, a larger air gap makes electric machine optimization of any kind more difficult, and larger size and weight are expected. Also, the regulation coil of the Raad Patent produces saturation of the armature. Operating points with saturation lead to poor utilization of the armature material, therefore further penalty to machine weight and volume is paid.
Still another permanent magnet power supply system is described in U.S. Pat. No. 5,254,936, issued to Leaf et al., the entire content of which is incorporated herein by reference. In the Leaf Patent, a dual generator system is shown wherein a pair of generators supply current to a common electrical load. The electric machine used in the Leaf Patent includes a field winding for excitation, therefore, the voltage regulation is achieved by controlling the current in the field winding. However, machines with such excitation windings are not suitable for direct integration with high-speed prime movers such as turbine engines. This is due primarily to structural limitations of the rotor.
Accordingly, a need exists for a PM machine which can provide constant DC voltage at variable prime mover speeds without using costly power electronics or adding to overall machine cost, weight and complexity.